PERC is a chemical known as perchloroethylene or tetrachloroethylene. It's the solvent used by about 85% of U.S. dry cleaners, but is also used as a metal degreaser and in the production of many other chemicals.

PERC is found in the air, in drinking water, and in soil. It can be detected in most people's blood, as well as in breast milk. What's the risk?

In 2008, the Environmental Protection Agency (EPA) suggested that PERC be classified as a "likely human carcinogen." Moreover, the EPA found that PERC's most dangerous noncancer toxicity is brain and nervous system damage -- and set safe exposure levels well below levels that cause such damage.

But rather than finalize the ruling, the EPA asked the prestigious National Academy of Sciences to review it's PERC risk analysis and to tell the EPA if it's system for analyzing chemical risk was correct.

Now the expert panel appointed by the National Academy of Sciences says the EPA was basically correct. The panel agreed that:

PERC is a "likely human carcinogen." This means that while there's no definitive proof that the chemical causes cancer in humans, there's strong evidence it does -- and there's proof that the chemical causes various cancers in animals.

PERC's most dangerous noncancer effect is nerve and brain damage. Safe exposure levels for drinking water and air quality should be set well above levels that can cause such damage.

The EPA's system for evaluating chemical risk is basically sound, although procedures for evaluating the strength of relevant studies need to be strengthened.

"We praised the EPA for doing a very thorough job," panel member Ivan Rusyn, MD, PhD, a toxicologist at the University of North Carolina, tells WebMD. "The overwhelming opinion of the committee was that the EPA was correct."

The major complaint the committee had with the EPA was that it put too much emphasis on a single study in setting the safe concentration level. That level is calculated by finding the highest dose that does no harm and dividing that dose by 1,000 or more to err on the side of safety.

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The EPA suggested that a safe PERC concentration would be 2 parts per billion. The National Academy of Sciences committee used several different studies to calculate a slightly higher safe level, between 6 and 50 parts per billion.

"This is an immaterial difference," Rusyn says.

The National Academy of Sciences panel ruled only on the science used by the EPA and did not offer any policy advice on the use of PERC by dry cleaners or other industries. Such policies are for the EPA, Congress, and the states to decide.

California, for example, in December 2007 passed a law that will outlaw the use of PERC in that state by the year 2023. The California law also requires removal of all dry cleaning machines 15 years old or older by July 1, 2010. As of July, no PERC machines may be used in buildings shared with California residences.

PERC is not the same chemical as perchlorate, a different environmental contaminant used in products such as rocket fuel and fireworks.

The Halogenated Solvents Industry Alliance Inc., a chemical industry group that has criticized the EPA's draft assessment of PERC, was unable to respond to WebMD's request for an interview in time for publication.